Feed's Effect on Horses' Dehydration Response

A research team concluded that providing food without water can actually increase dehydration in horses.

Photo: The Horse Staff

Horse owners are fully aware of the importance of free-choice water in a horse’s diet. However, in some situations, such as during transportation for domestic horses or during droughts for wild horses, horses might have access to feed but not water. How do these situations affect a horse’s hydration status?

A group of researchers led by Moira Norris, DVM, of the Cornell University College of Veterinary Medicine, set out to answer this question when they conducted a study evaluating horses' fluid balance changes due to water deprivation, with and without available feed.

The team employed six Shetland-type pony geldings in their study, housing each in a stall for 24 hours during each experiment to allow for urine collection and returning them to pasture between experiments. The team subjected each pony to each of the following treatments:

Control (both water and food available ad libitum, WWWF)

No water but with food (NWWF)

No water and no food (NWNF)

Water but no food (WWNF)

The team offered ponies in the food groups free-choice hay and ponies in the water groups a water bucket. They measured the amount of water each pony consumed and collected blood samples hourly up until 22 hours and whenever the pony urinated. For Hours 23 and 24, the team only collected blood samples when the pony urinated.

The team analyzed the blood samples for packed cell volume (PCV) and plasma protein concentration, both of which they used to estimate blood volume changes. They analyzed blood and urine samples for osmolality and sodium and potassium levels to determine fluid and electrolyte balance change versus time zero.

Upon reviewing their study results, the team found that ponies drank more water (13.1 kg) when offered food compared to when not offered food (3.5 kg). Simply put, “Horses drink more when food is available and eat more when water is available,” said researcher Katherine Houpt, VMD, PhD, a professor emeritus at Cornell University. The team believes this is because the digestive system releases isotonic secretions when food is offered, so there is less fluid available as circulating blood.

The team also determined that:

Treatment significantly affected changes in fluid and electrolyte balance. The team noted the most significant plasma osmolality change after 22 hours of treatment in the NWWF group, compared to the other treatment groups. The team explained that the ponies in that group appeared to be physiologically “more dehydrated” when food was provided without water. The team hypothesizes that osmolality could be a primary stimulus to drink when food is present, as plasma osmolality changes remained significantly lower when NWWF was compared to WWWF.

Plasma osmolality decreased in both WWNF and NWNF treatments. In contrast to dehydration, low osmolality stimulates a decrease in water absorption and an increase in water excretion, meaning the ponies in these groups were physiologically responding as if they were hydrated, even when water was not present.

PCV declined when food was not available to the ponies and in all treatments during the evening hours when food and water intake slowed. Additionally, by Hour 22, NWNF ponies had significantly lower PCV compared to NWWF and WWWF. The researchers explained that PCV is a measure of the percentage of red blood cells in a whole blood sample; in equids, the spleen releases red blood cells in response to a meal. When the ponies weren't offered food, the team believes the spleen removed red blood cells from the circulation for storage or dilution of the PCV by an increase in plasma volume.

There were no significant differences in blood volume during the 24-hour period for all treatment groups. The team believes the ponies maintained this balance even without water intake due to the large amount of fluid contained in the hindgut.

Ponies' total urine volumes during the 24-hour treatment periods were similar in all groups and averaged seven to eight liters per day. Osmolyte excretion, however, was affected by treatment, and water clearance was higher in WWWF, NWNF and WWNF conditions compared to the NWWF group. This suggests a vasopressin effect, meaning the body was retaining more water and constricting the blood vessels, the team noted.

Take-Home Message

The team found that the ponies in this study could mitigate dehydration when not provided with water by possibly increasing fluid absorption from the hindgut and reducing water loss through urine. With the addition of food, however, it appeared these physiological changes were more difficult for the ponies to make, which could negatively impact their ability to respond to dehydration. In simple terms, the team concluded that providing food without water can actually increase dehydration in horses.

The best way to combat dehydration is to prevent it from happening in the first place by always ensuring horses have access to a clean water source.

About the Author

Kristen M. Janicki, a lifelong horsewoman, was born and raised in the suburbs of Chicago. She received her Bachelor of Science degree in Animal Sciences from the attend the University of Illinois at Urbana-Champaign and later attended graduate school at the University of Kentucky, studying under Dr. Laurie Lawrence in the area of Equine Nutrition. Kristen began her current position as a performance horse nutritionist for Mars Horsecare, US, Inc., and Buckeye Nutrition, in 2010. Her job entails evaluating and improving the performance of the sport horse through proper nutrition.

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